(i) Field of the Invention
The present invention relates to a fabrication method of a solid electrolytic capacitor and, more particularly, to a fabrication technique that is effective in decreasing both of a leakage current and an equivalent series resistance in a solid electrolytic capacitor using a conducting polymer as a solid electrolyte.
(ii) Description of the Related Art
In a solid electrolytic capacitor obtained in such a manner that a solid electrolyte is formed on a dielectric film obtained by anodically oxidizing valve metal such as tantalum or aluminum, recently, a conducting polymer such as polypyrrole, polythiophene, or polyaniline is used as a solid electrolyte in many cases. The conducting polymer has an equivalent series resistance (ESR) that is lower than that of manganese dioxide used popularly as a solid electrolyte so far, and is excellent in characteristics in high-frequency regions.
The solid electrolytic capacitor using the conducting polymer is a capacitor that has a low ESR and is excellent in characteristics in the high-frequency regions. On the other hand, however, has been known such a fact that the leakage current is relatively higher than that of the conventional solid electrolytic capacitor using manganese dioxide. Hitherto, to improve such leakage current characteristics, for example, in a technique disclosed in Japanese Patent Application Laid-Open Publication No. 11-087187/1999, after the solid electrolyte of the conducting polymer is formed on the dielectric film by a chemical polymerization, the dielectric film is subjected to an anodic reoxidation (hereinbelow, referred to as a reformation). It is considered that the reformation heals defective portions of the dielectric film to reduce the leakage current.
A fabrication method of a tantalum solid electrolytic capacitor disclosed in the above-mentioned Publication will now be described with reference to FIGS. 1 and 2A and 2B. The fabrication method of the solid electrolytic capacitor in which the solid electrolyte of the conducting polymer is formed on the dielectric film and, after that, the dielectric film is reformed will now be explained by using a tantalum solid electrolytic capacitor as an example.
FIG. 1 is a flowchart showing a fabrication process of the tantalum solid electrolytic capacitor having a reforming step in accordance with the order of processing steps. FIG. 2A is a cross-sectional view of a capacitor element at the point of time when the processes up to the formation of a conducting polymer layer in the fabrication of the tantalum solid electrolytic capacitor are finished. FIG. 2B is a schematic view for explaining a forming method.
First, fine powder of metal tantalum is pressed to be molded into a cylindrical or prismatic form. The molded article is sintered, thereby obtaining a micro-porous sintered body 1 (step S10). At that time, a tantalum wire 2 is prepared before the molding. The tantalum wire 2 is set into one plane of the molded article upon molding.
Subsequently, as shown in FIG. 2B, the sintered body 1 is immersed in a forming solution 7 such as an aqueous solution of phosphoric acid. A high-order potential is applied to the tantalum wire 2 of the sintered body 1 and a low-order potential is applied to an opposite electrode 5 to perform an anodic formation, thereby forming a tantalum oxide film 3 as a dielectric on the surface of the micro-porous surface of the sintered body 1 (step S20). The sinter 1 on which the tantalum oxide film 3 has been formed will be called a formation, hereinbelow.
Performing a chemically-oxidative polymerization forms a polypyrrole layer 4 as a conducting polymer layer on the tantalum oxide film 3 of the foregoing formation (step S30).
After that, in a manner similar to the above-mentioned forming process (step S20), the tantalum oxide film 3 is reformed (step S40). The thickness of the polypyrrole layer 4 is adjusted to a predetermined thickness by repeating the steps S30 and S40 several times.
Due to the reformation in step S40, defective portions in the tantalum oxide film 3 as a dielectric are oxidized so as to be healed, and the polypyrrole layer 4 in the portion corresponding to a path for a current during the reformation is partially oxidized to become an insulator. Accordingly, a leakage current (LC) in the case where the article is used as a capacitor is reduced.
After that, a graphite layer and a silver paste are formed so as to be laminated on the polypyrrole layer 4 in accordance with this order to provide a cathode conductive layer (not shown) (step S50). Subsequently, an external cathode terminal is fixed to the cathode conductive layer by a conductive adhesive and an external anode terminal is fixed to the tantalum wire 2 by welding, so that the two external terminals of the cathode and anode are attached (step S60). After that, the resultant article is sheathed by a transfer molding process using an epoxy resin (step S70), so that the tantalum solid electrolytic capacitor is completed.
Ordinarily, when the tantalum oxide film as a dielectric is reformed, the conductive polymer layer is formed due to the chemically-oxidative polymerization in order to prevent the forming solution from becoming dirty and, after that, the layer is cleaned by using water or alcohol. Although an alcohol cleaning is effective, the cost of chemical is high. Accordingly, a water washing is generally performed. Therefore, it is thinkable that the conducting polymer layer before the reformation contains moisture during water washing. When the conducting polymer layer is formed by the chemically-oxidative polymerization and, after that, a conducting polymer layer is thickly applied on the above layer by an electrolytic polymerization, moisture is also trapped in the conducting polymer layers.
The inventors of the present invention found that as a problem in the case where the reformation was performed without removing the trapped moisture, the equivalent series resistance ESR of the capacitor was raised.
It is, therefore, an object of the present invention to provide a fabrication method of a solid electrolytic capacitor, comprising a step of performing a reformation after the formation of a conducting polymer layer, wherein an equivalent series resistance is not deteriorated due to the reformation.
According to the present invention, the fabrication method of the solid electrolytic capacitor is characterized in that the conducting polymer layer is dried before the reforming step to decrease moisture in the conducting polymer layer to a predetermined amount and, after that, an oxide film made of valve metal is reformed.
According to the present invention, after the conducting polymer layer is formed on the oxide film of the valve metal, the moisture in the conducting polymer layer is decreased to a predetermined amount. Consequently, it is possible to suppress such a phenomenon that the conducting polymer layer is reacted with water in the conducting polymer layer by a voltage that is applied to the conducting polymer layer at the time when the oxide film of the valve metal is reformed, so that the polymer layer is excessively insulated, and it is possible to prevent an increase in equivalent series resistance of the solid electrolytic capacitor. According to the present invention, simultaneously with the prevention of the increase in equivalent series resistance of the solid electrolytic capacitor, in a manner similar to the related art, the leakage current in the solid electrolytic capacitor can be also reduced.